Mandy - thanks for the plug, yes we do courses and a book see our site
www.oaklandspigs.co.ukIn terms of financials, the question is not so much how many pigs you can keep in 4 acres (and it is a lot!), but what you will do with them.
Unfortunately nowadays selling finished pigs to a third party (market, abattoir, butcher) you will almost certainly make a loss ie you will spend more money feeding and keeping than you'll get for selling either piglets or finished pigs.
Selling pork to family and friends - you should be able to sell half a pig and get the other half for yourself free !
But how much pork can you sell to family and friends.
Selling pork to public - lots of time and regulation involved.
So I'd start the other way round - how many pigs can I sell? Then consider subdividing to have areas you work and areas you rest.
oh and yes, keep clear of the river - pigs swim very well ! and you'll want to avoid them fouling it.